Ambient air quality can affect people's health. The lower the air quality, the greater the risk for health-related problems induced by the ambient air.
One parameter of air quality is the amount of particulate matter present in an air sample, which can originate from anthropogenic emissions (e.g., power generation, metal refineries, cement production, and waste incineration) or atmospheric formation from gaseous precursors. Conventional particulate matter sampling devices can be used to measure a mass concentration of particulate matter within ambient air, source emissions, gases, or other fluids to determine ambient air quality. A conventional particulate matter sampling device typically can provide a warning to a user when the device detects a condition of relatively low air quality (e.g., a relatively large particulate mass concentration within the air) or a decrease in the ambient air quality based upon an increase in particulate mass concentration measured over a particular time period.
Continuous particulate monitoring in stack effluent streams is becoming a growing concern in the US as well as abroad. Accordingly, stricter standards have been or will be implemented to cut back on such pollution. As a result of tightening standards, new methods are being sought to accurately measure the primary particulate emissions from industrial sources using a direct monitoring device.
Several sensing techniques provide continuous direct monitoring of particulate mass concentration. For example, the sensing techniques used for particulate monitoring include mass sensing methods such as an inertial mass measurement instrument (i.e., tapered element oscillating microbalance or TEOM), beta radiation attenuation and optical sensing methods such as light scattering photometry or nephelometry.
In a conventional inertial mass measurement instrument, an oscillating element in a microbalance is provided with a filter on its oscillating end for the entrapment of matter whose mass is to be determined by passing the medium containing such matter therethrough. The oscillating element itself is hollow and the medium passes first through the filter and then through the oscillating element. The measurement of mass can be calculated based on a change in the oscillation frequency. As the collection of aerosol accumulates on the filter, the mass increases, thereby decreasing the frequency of oscillation. By measuring only the change in frequency, one can determine the gain in the aerosol mass on the collection medium.
Beta radiation attenuation devices typically include a mass sensing stage and a particle collection stage. The mass sensing stage includes a beta particle radiation source, typically carbon-14 or krypton-85, and a beta particle detector, typically a Geiger-Muller detector, plastic scintillator, proportional counter or an ionization chamber. The particle collection stage typically includes a filter and vacuum source. The mass sensing stage will position the filter between the beta particle radiation source and the beta particle detector. Some devices are known to combine the stages for simultaneous mass collection and mass sensing. Beta radiation attenuation devices exhibit a substantially exponential attenuation characteristic as a function of the mass per unit area collected by the filter between the radiation source and the radiation detector. For example, during operation, ambient air (or another gas) flows through the filter and the filter collects particulate matter present within the ambient air over time. As the amount of particulate matter collected by the filter increases, the particulate matter attenuates the beta particles emitted from the radiation source (i.e., the beta particle detector senses less radiation from the beta source) as detected by the detector. Because the attenuation of the beta radiation detected by beta particle detector is related to the mass of the particulate matter collected by the filter and does not substantially depend upon the type or compound of material collected by the filter, a beta radiation signal produced by the beta radiation attenuation device indicates a particulate mass concentration of particulate mater within an air sample.
Conventional beta radiation attenuation devices, and other mass sensing devices that utilize a filter to collect particulate matter within an air sample, sometimes utilize temperature and humidity conditioning elements to remove liquid water from the air sample. Should the filter collect liquid water over time, the liquid water will be measured as mass and can affect the accuracy of the device's detection of particulate mass within the air sample.
In conventional particulate monitoring devices, prior to the air sample reaching the filter, the device will either reduce the sample relative humidity by applying heat, use a permeation drying technique to remove water content from the sample stream, or dilute the sample with clean dry air of a considerably lower dewpoint. By reducing the humidity or percentage of water content within the air sample the particulate matter detection accuracy is improved.
Another type of conventional particulate mass sensing device is a light scattering photosensitive device called a nephelometer. Light scattering photometry devices, such as nephelometry devices, measure the irradiance of light scattered by particles passing through a sensing volume. Typical light scattering photometry devices include an incident light beam and detection optics or sensors that measure the strength of the light beam and the intensity of the light scattered by the particles and carrier gas. During operation, ambient air or another gas flows through a sensing volume defined by an intersection of the illuminating beam and the field of view of the sensing optics. As the air flows through the sensing volume, the light scattering photometer illuminates particles present within the sensing volume and the optics and associated photosensitive measuring circuitry detect the light scattered by the particles. For an ambient air sample having a fixed size distribution of particles with invariant density and index of refraction, the intensity of light scattered by the particles within the air sample is directly proportional to the mass concentration of the particles within the air sample. Light scattering photometry or nephelometry devices, therefore, allow real-time (i.e., substantially instant) measurement of particulate mass concentration of ambient air.